The present invention relates to a system for monitoring the field winding of a rotating electric machine, particularly for the purpose of detecting the presence and location of a ground fault.
The field winding of a rotating electric machine, such as a synchronous machine, is designed and installed to be substantially fully insulated from ground during operation. However, a ground fault can develop at any time in the winding, due to an insulation breakdown, for example. It is important that the machine operator become immediately aware of such ground fault condition so that the machine can be shut down in order to perform appropriate repair. Although a single ground fault may not cause damage, the occurrence of a second ground fault can result in the flow of extremely large currents, causing severe machine damage.
Various systems for detecting such ground faults are already known. Thus, there exist AC detection systems in which an AC voltage is supplied between ground and one of the field winding terminals through a resistor. If no significant current flow through the resistor is detected, it can be concluded that no field ground exists. However, if a field ground does occur at any point along the length of the field winding, the current flow through the resistor will provide an indication of this condition.
According to one known DC detection method, two fixed value resistors are connected in series across the field winding so that the potential drop across the winding also appears across the resistors. The point of connection between the two resistors is connected to ground through a further resistor and a current detector. Under normal conditions, when the field winding is energized and no ground fault exists, substantially no current will flow through the further resistor and the current detector. However, if a ground fault should occur, the resulting current flow through the detector will provide an indication of that condition. However, this arrangement will not produce an indication if the ground fault occurs in a region of the field winding which, during normal operation, is at a potential corresponding to that of the connection point between the two series resistors. In order to deal with this problem, the circuit includes an auxiliary resistor which the operator can connect across one of the series resistors by means of a push button in order to temporarily shift the potential at the point of connection between the series resistors.
According to another known technique, which is comparable to the DC method described above, one of the series resistors has a nonlinear resistance variation with voltage so that as the field winding voltage varies, the potential at the point of connection between the series resistors also varies, thereby helping to detect a ground at any point along the length of the field winding. However, there are certain operating conditions, such as when the machine is base loaded, under which the winding voltage does not vary significantly over long periods of time.
The AC detection method has the disadvantage that it imposes an AC voltage on the field winding and this voltage can induce varying levels of non-ground currents due to the normal capacitive coupling of the field winding to ground. While the known DC detection methods overcome this drawback, they cannot, without operator intervention, detect ground faults at every point along the length of the field winding.
Moreover, none of the known techniques provides any information as to the location of a ground fault. If the existence of a ground fault is observed, using these known techniques, it is necessary to shut the machine down to locate that fault. However, this is not always possible because there are situations in which a ground fault may be apparent while the machine is in operation, but cannot be located after the machine has been shut down.